1. Field of the Invention
The present invention relates to communications systems. More specifically, the present invention relates to GPS positioning systems and wireless networks.
2. Description of the Related Art
The trend in the wireless communications industry is to provide a service that generates accurate position information for wireless terminals and to provide this information to requesting entities. This trend is driven to a large extent by the needs of public safety service providers in their efforts to promptly respond to emergency calls. In many instances, the calling party may be unwilling or unable to provide accurate position information. When such information is provided automatically, as is the case in wireline telephony services, public safety officials are able to respond and render service quickly. In general, the place that a public safety entity receives ‘911’ calls is known as the Public Safety Answering Point (hereinafter ‘PSAP’).
In wireless telephone networks, such as cellular or PCS networks, the provision of automatic calling party position information is more difficult than in wireline telephony networks because of the inherent mobility of wireless telephones. In some wireless systems, the PSAP is provided with position information that resolves which wireless base station, or perhaps which radio sector within a wireless base station, is handling the emergency call. Position information to this degree of resolution only identifies the calling party location to a rather broad geographic region, so the PSAP dispatchers have to rely on the position information given orally by the calling party before they can respond to a emergency service request.
The Federal Communications Commission (hereinafter ‘FCC’) forced the market to address the position information provision issue in June 1996 when it adopted a report and order for enhanced E911 wireless service. On Dec. 23, 1997, the FCC issued a revised version of the report and order. Highlights are as follows:
Within twelve months after the effective date of the rules, the FCC will require that cellular, broadband PCS and geographic area Specialized Mobile Radio (hereinafter ‘SMR’) systems transmit to the PSAP all E911 emergency calls from any mobile station that transmits a mobile identification number (hereinafter ‘MIN’), or its functional equivalent, without any interception by the carrier for credit checks or other validation procedures.
Beginning twelve months (and completed within eighteen months) after the effective date of the rules, the FCC requires cellular, broadband PCS and geographic SMR licensees to offer certain E911 enhancements. These E911 enhancements include the ability to relay a caller's telephone number (call back the E911 caller if a call is disconnected). Also, carriers must be capable of routing E911 calls to an appropriate PSAP.
Within five years after the effective date of the rules, the location (position) of the mobile station making the emergency call must be provided to the qualified PSAP in two-dimensions and have an accuracy within a 125 meter radius measured using root mean square (RMS) methods. According to the FCC, a request is qualified if and when (1) a PSAP indicates it has the capability to receive and utilize the number and location passed along by the wireless carrier, and (2) there is a cost-recovery mechanism in place.
The FCC position accuracy requirements are a minimum so suppliers and manufacturers of wireless network equipment are working to provide location data that is more accurate than the minimum. For example, U.S. Pat. No. 6,021,330 to Vanucci for MOBILE LOCATION ESTIMATION IN A WIRELESS SYSTEM USING DESIGNATED TIME INTERVALS OF SUSPENDED COMMUNICATIONS, assigned to Lucent Technologies, teaches a system wherein the location of mobile stations is estimated through measurement of differential path delay times of beacon signals synchronously transmitted by several base stations. A trilateration calculation is made to determine position.
Another approach to location measurement of a mobile station in a wireless network is taught by a patent assigned to Qualcomm Inc.: U.S. Pat. No. 6,081,299 to Soliman et al., for SYSTEM AND METHOD FOR DETERMINING THE POSITION OF A WIRELESS CDMA TRANSCEIVER. Soliman et al. teach a more sophisticated approach to mobile station location determination utilizing both Global Positioning System satellite and terrestrial base station signals. Generally, Solimon et al. teach the process of receiving at a base station a first signal transmitted from a first GPS satellite and a second signal transmitted from a second GPS satellite. The mobile station is adapted to receive these GPS signals as well and transmit a third signal to the base station in response thereto. The base station receives the third signal and uses it to calculate the position of the mobile station. In one specific implementation, the base station sends aiding information to the mobile station. The aiding information is used by the mobile station to more quickly acquire the signals transmitted by the first and second satellites than is possible without the presence of such aiding information.
The use of GPS satellite position and velocity measurement is a good approach to wireless terminal location determination because positions can be determined within the accuracy requirements of the FCC report and order. It also has other benefits in that new GPS features can be integrated into a wireless telephone once the GPS technology has been add added to the unit. These extra value features can be used to increase the market value of the product and to enhance revenues through the provision of additional services to the end users of such products.
The GPS navigation system employs satellites that are in orbit around the Earth. Any user of GPS, anywhere on Earth, can derive precise navigation information including 3-dimensional position, velocity and time of day. The GPS system includes 24 satellites that are deployed in circular orbits with radii of 26,600 kilometers in three planes inclined at 55° with respect to the equator and spaced 120° with respect to one another. Eight satellites are equally spaced within each of the three orbit paths. Position measurements using GPS are based on measurements of propagation delay times of GPS signals broadcast from the orbiting satellites to a GPS receiver. Normally, reception of signals from 4 satellites is required for precise position determination in 4 dimensions (latitude, longitude, altitude, and time). Once the receiver has measured the respective signal propagation delays, the range to each satellite is calculated by multiplying each delay by the speed of light. Then, the location and time are found by solving a set of four equations with four unknowns incorporating the measured ranges and the known locations of the satellites. The precise capabilities of the GPS system are maintained by means of on-board atomic clocks for each satellite and by ground tracking stations that continuously monitor and correct satellite clock and orbit parameters.
Each GPS satellite transmits two direct-sequence-coded spread spectrum signals in the L-band. An L1 signal at a carrier frequency of 1.57542 GHz, and an L2 signal at 1.2276 GHz. The L1 signal consists of two phase-shift keyed (PSK) spread spectrum signals modulated in phase quadrature. The P-code signal (P for precise), and the C/A-code signal (C/A for coarse/acquisition). The L2 signal contains only the P-code signal. The P and C/A codes are repetitive pseudo-random sequences of bits (termed “chips” by spread spectrum engineers) that are modulated onto the carriers. The clock-like nature of these codes is utilized by the receiver in making time delay measurements. The codes for each satellite are unique, allowing the receiver to distinguish which satellite transmitted a given code, even though they are all at the same carrier frequency. Also modulated onto each carrier is a 50 bit/sec data stream that contains information about system status and satellite orbit parameters, which are needed for the navigation calculations. The P-code signals are encrypted, and are not generally available for commercial and private users. The C/A signal is available to all users.
The operations performed in a GPS receiver are for the most part typical of those performed in any direct-sequence spread spectrum receiver. The spreading effect of the pseudo-random code modulation must be removed from each signal by multiplying it by a time-aligned, locally-generated copy of the code, in a process known as despreading. Since the appropriate time alignment, or code delay, is unlikely to be known at receiver start-up, it must be determined by searching during the initial “acquisition” phase of a GPS receiver's operation. Once determined, proper code time-alignment is maintained during the “tracking” phase of GPS receiver operation.
Once the received signal is despread, each signal consists of a 50 bit/sec PSK signal at an intermediate carrier frequency. The exact frequency of this signal is uncertain due to the Doppler effect caused by relative movement between satellite and terminal unit, and to local receiver GPS clock reference error. During initial signal acquisition this Doppler frequency must also be searched for, since it is usually unknown prior to acquisition. Once the Doppler frequency is approximately determined, carrier demodulation proceeds.
After carrier demodulation, data bit timing is derived by a bit synchronization loop and the data stream is finally detected. A navigation calculation may be undertaken once the signals from 4 satellites have been acquired and locked onto, the necessary time delay and Doppler measurements have been made, and a sufficient number of data bits (enough to determine the GPS time reference and orbit parameters) have been received.
One drawback of the GPS system for location determination is the long time needed for the initial signal acquisition phase. As mentioned above, before the four satellite signals can be tracked they must be searched for in a two-dimensional search “space”, whose dimensions are code-phase delay, and Doppler frequency shift. Typically, if there is no prior knowledge of a signal's location within this search space, as would be the case after a receiver “cold start”, a large number of code delays (about 2000) and Doppler frequencies (about 15) must be searched for each satellite that is to be acquired and tracked. Thus, for each signal, up to 30,000 locations in the search space must be examined. Typically these locations are examined one-at-a-time sequentially, a process which can take 5 to 10 minutes. The acquisition time is further lengthened if the identities (i.e., PN-codes) of the four satellites within view of the receiving antenna are unknown.
In the case where a GPS receiver has already acquired the satellite signals and is then in tracking mode, the position determination process is virtually instantaneous. However, in the routine use of wireless terminals, users turn the power on and quickly begin operation. This may be the case when an emergency communication is intended. In such situations, the time delay associated with a 5 to 10 minute GPS satellite signal acquisition cold-start by a GPS/wireless terminal unit before a position fix can be obtained limits the response time of the system.
Thus, a need remains in the art for a system and method for decreasing the time required to acquire GPS satellite signals and render a position fix in a GPS/wireless terminal unit.